The present invention relates to an improved electromagnetic spring-wound clutch which may be advantageously employed to connect an air conditioning compressor to an engine of an automotive vehicle. This invention constitutes a novel and advantageous improvement to an ELECTROMAGNETIC SPRING-WOUND CLUTCH disclosed in copending U.S. patent application Ser. Nos. 866,645, filed Jan. 3, 1978 (now U.S. Pat. No. 4,194,607, issued Mar. 25, 1980), 917,316, filed June 19, 1978 and 942,262, filed Sept. 14, 1978, (now U.S. Pat. No. 4,225,027, issued Sept. 30, 1980) which are assigned to the same assignee as this application.
An electromagnetic spring-wound clutch of the general type to which the present invention relates is disclosed in U.S. Pat. No. 3,735,847 and comprises coaxial input and output members. A clutch plate or disc is rotatably provided on the input member. Furthermore, a coil-wound spring is wound around the input and output members and connected at its opposite ends to the clutch disc and output member. An electromagnetic coil attracts the clutch disc into frictional engagement with the input member, causing the coil-wound spring to be tightly wound around the input and output members and drivably connect the input member to the output member. When the coil is de-energized, the clutch disc disengages from the input member, and the coil-wound spring releases the input member for rotation relative to the output member.
A major problem in this type of clutch is obtaining fast and smooth disengagement of the clutch disc. More specifically, the clutch disc tends to vibrate during disengagement, causing noise, and furthermore does not disengage completely parallel to the input member. In extreme cases, the clutch disc may be deformed or even break. A prior art attempt to overcome this problem involves an arrangement in which the coil-wound spring is stretched axially by the clutch disc when the coil is energized, and disengages the clutch disc when the coil is de-energized due to its resilience. However, due to the large spring constant required to drivably connect the input member to the output member, the coil must have an excessive magnetomotive force to stretch the coil-wound spring. Such a large coil cannot be fit into the clutch in a practical manner.
The problem is basically overcome by the novel improvement disclosed in the above mentioned U.S. patent application Ser. No. 866,645 by providing spring means and a plurality of pins to quickly and smoothly disengage the clutch disc from the input member while maintaining the clutch disc parallel to the input member. The spring means in one preferred form of the prior disclosure comprises an annular plate spring having axially projecting tabs or an annular wave spring. The clutch comprises a cover which protectively surrounds the coil-wound spring and has a radially outer flange portion which is formed with annular slots through which the pins extend. The pins further extend through holes formed through the clutch disc and annular spring and are formed with double heads which resiliently compress the spring and urge the clutch disc away from engagement with the input member and into engagement with the flange portion of the cover.
Although the basic improvement provides a substantial advance over the prior art, a problem has been encountered in actual manufacture of such a clutch. In order to enable deep drawing of the cover into the required shape, the cover must be made of a relatively soft low carbon steel. Due to the fact that the clutch disc and therefore the annular spring rotate relative to the cover during engagement and disengagement of the clutch, the engaging portions of the cover and annular spring rub together. Whereas there is but slight abrasion of the annular spring, which is made of a high carbon spring steel, frictional contact with the annular spring causes substantial abrasion of the cover and the formation of deep groove over a period of prolonged use of the clutch. This progressively reduces the force of the annular spring exerted on the clutch disc and the reliability of smooth disengagement of the clutch. After excessive use the clutch may finally fail to disengage.
Such abrasion of the cover may be eliminated by forming the cover of a high carbon steel and hardening the same by a quenching process. However, it is impossible to deep draw such a high carbon steel into the required shape for the cover. Another unsatisfactory expedient is to form the cover of a relatively low carbon steel and surface harden the same after press forming. However, practical problems are encountered regarding deformation of the cover during surface hardening and the additional manufacturing step which increases the production cost.